1. Field of the Invention
The present invention relates to a semiconductor light emitting device, in more particularly to a semiconductor light emitting device, in which a current spreading property can be improved without increasing a thickness of an active layer, a thickness of a semiconductor layer and an area of a first electrode.
2. Related Art
As to a light emitting diode (LED) which is a semiconductor light emitting device, it is possible to fabricate a high luminance LED emitting various color lights such as blue, green, orange, yellow, and red, since it is possible to grow a GaN based high quality crystal or an AlGaInP based high quality crystal by using MOVPE (Metal Organic Vapor Phase Epitaxy) method in recent years. In accordance with the provision of a high luminance light emitting diode, applications of the LED are widened, e.g. a back light of a liquid crystal display, a brake lump for a vehicle, so that demand for the LED increases year by year.
Since the growth of a high quality crystal by the MOVPE method has been achieved, a light emitting efficiency inside the light emitting device is nearing to a theoretical limit value. However, the light extract efficiency from the light emitting device to the outside is still low, and enhancement of the light extract efficiency is expected.
For example, a high luminance red LED is made from AlGaInP based materials, and has a double hetero structure comprising a conductive GaAs substrate, an n-type AlGaInP layer comprising an AlGaInP based material with a composition which is lattice-matched with the conductive GaAs substrate, a p-type AlGaInP layer, and an active layer which is a part of a light emitting part comprising AlGaInP or GaInP, and the active layer is sandwiched by the n-type AlGaInP layer and the p-type AlGaInP layer. The AlGaInP based material here is a general term of various kinds of materials mainly comprising AlGaInP, in that composition ratios or additives are different from each other. In the semiconductor light emitting device using AlGaInP based material, materials such as GaInP, GaP may be used together.
Since a bandgap of the GaAs substrate is narrower than that of the active layer in such a semiconductor light emitting device, most of the light emitted from the active layer is absorbed by the GaAs substrate, so that the light extract efficiency is deteriorated.
As means for solving this problem, there is a technique for improving the light extract efficiency by forming a layer with a multilayer reflective film structure comprising semiconductors having different refractive indices between the active layer and the GaAs substrate to reflect the light emitted to the GaAs substrate, thereby reducing an absorption of the light in the GaAs substrate. However, according to this technique, only the light having a limited incident angle with respect to the multilayer reflective film structure layer is reflected. In other words, only a part of the light emitted to the GaAs substrate is reflected, so that it is difficult to improve the light extract efficiency enough.
Thus, Japanese Patent Laid-Open No. 2002-217450 discloses another technique for realizing a high luminance by manufacturing a semiconductor light emitting device in which a double hetero structure part comprising AlGaInP based material is grown on a GaAs substrate for growth, sticking the double hetero structure part on a supporting substrate such as Si or GaAs thereafter via a metal layer with a high reflectance, and removing the GaAs substrate used for the growth. According to this technique, since the metal is used as a reflective layer, the reflection with high reflectance can be realized without selecting an incident angle with the reflective layer. For this reason, it is possible to provide a higher luminance than the aforementioned technique in which the multilayer reflective film structure is formed. In other words, it is possible to achieve the higher luminance by extracting the light generated in the active layer more effectively.
In the conventional semiconductor light emitting device, a thickness of the semiconductor layer (epitaxial layer) is increased to improve the current spreading, thereby realizing the high luminance. However, in the semiconductor light emitting device in which the high luminance is achieved by effectively reflecting a light at a reflective metal film, a step for removing and sticking a substrate is required, so that the thickness of the semiconductor layer cannot be much increased. When the thickness of the semiconductor layer is increased, a warping of the epitaxial wafer is also increased, so that it is difficult to conduct the step of removing and sticking the substrate, thereby deteriorating a production yield.
Further, there is a disadvantage in a material cost when the thickness of the semiconductor layer is increased.
So as to solve this problem, as a technique for improving the current spreading (current dispersion) without increasing the thickness of the semiconductor layer, a configuration in which a first electrode comprises a narrow wire electrode is proposed. According to this structure, the current spreading can be improved in accordance with an increase in an area of the first electrode by using the narrow wire. However, there is disadvantage in that the light is difficult to be extracted to the outside, since an area of a shadow of the first electrode is increased. Namely, there is a trade-off relationship between the current spreading and the light extract. In more concrete, the change in the area of the first electrode is affected by the trade-off relationship between the improvement in the current spreading and the increase in the shadow of the first electrode.
In the semiconductor light emitting device, in which the high luminance is achieved by effective reflecting the light at the reflective metal film, the light emitted from the active layer is reflected at the reflective metal film and transmitted again through the active layer. However, since the active layer is not perfectly transparent with respect to the emitted light, the light reflected at the reflective metal film is absorbed in the active layer to some extent. So as to reduce the absorption of the light, it is desirable to reduce a thickness of the active layer. However, when the thickness of the active layer is excessively reduced, there is a disadvantage in that the light output is deteriorated due to an overflow of carriers.